I've seen it happen twice in the past year.

A few months ago I was flying DTW to LAS on a Friday night and we had left the gate awaiting to take off and then the pilot made an announcment we had to return to the gate. I thought at first it was a mechanical problem but the engines weren't shut off and a few minutes later 2 police officers came on board and removed 2 passangers and an FA told people sitting in first that a fight broke out between 2 passengers in coach and an FA sitting in the back wanted them removed so they returned to the gate. I was sitting row 7 in first and couldn't see back into coach from my seat but didn't hear any noise at all.

Another time on a MDW/DTW RJ we were on the ground for a while with a weight issue at MDW and the seatbelt sign never went off the entire flight and we had to wait probably 45 minutes for a gate once we landed at DTW. A pax got out of his seat when we were on the ground at DTW waiting on our gate and told the FA he had to go to the bathroom really bad and couldn't hold it any longer and she told him to sit down and he did and he asked her to to ask the pilot how much longer because he really had to go. She did call into the cockpit and he gave the ok for him to go so he got up and went to the lav and came back. When we got to the gate finally the pilot asked everyone to stay seated for a few more minutes and then 2 police officers came on board and told the passenger the pilot wanted him removed and several passengers said he did nothing wrong. I think it was a FA on a power trip. VERY lazy FA, she get out of her seat the entire flight even to collect trash. When I got off the plane the officers were talking to him so not sure what happened.